Stabilized vacuum tube oscillator



March 10, 1942. L.v A. MEACHAM STABILIZED VACUUM TUBE OSCILLATOR Filed June 24, 1939 2 Sheets-Sheet l LEBD IVY

INVENTOR L. A. MEAGHAM ATTORNEY March 10, 1942. L. A. MEACHAM 2,275,452

STABILIZED VACUUM TUBE OSCILLATOR Filed June 24, 1939 2 Sheets-Shet 2 FIG. 2

LOCUS OF END OF M VECTOR FOR VARY/N6 FREQUENCY w/ mour PHASE- come-C TING NE TWO/PK WITH PHASE- COPREC T/NG NE TWORK INVENTOR By .AMEACHAM W A T TORNE V v Patented Mar. 10, 1942 2,275,452 I STABILIZED VACUUM TUBE OSCILLATOR Larned A. Meacham, Verona, N. 1., assignor to Bell Telephone laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 24, 1939, Serial No. 280,897

12 Claims.

This invention relates to vacuum tube oscillation generators of the feedback type and, specifically, represents an improved and more highly elaborated form of the prototype circuit as disclosed in applicants Patent 2,163,403, having issue date of June 20, 1939.

It is the principal object of the invention to still further improve the operating characteristics of such circuit with respect both to frequency stability and amplitude stability of the generated oscillations. A subsidiary object is the further improvement of the wave form of the resultant oscillations. Experiment has shown that the generic circuit, as matured into the form represented by the present invention, has a frequency stability many times as great as that of the circuit of the above-identified patent.

The prototype circuit evinced a recognition of the fact that the frequency of oscillations generated in vacuum tube oscillator circuits is de-' pendent, in the general case, on the resistances of the grid and plate space paths of the tube and that one of the principal causes of frequency instability in the presence of variation of the tube excitation voltages is the non-linear character of these resistances, these non-linear resistances being also productive of distortion of the wave form and the generation of harmonic frequencies.

A remedy was found in the use of a special form of feedback circuit which makes the oscillation frequency substantially independent of the vacuum tube resistances and of variations of any impedance element in the system other than a particular frequency determining combination.

The amplitude of the oscillations is regulated and maintained substantially constant by auto-'- matic control of the magnitude of the feedback, which makes it possible to operate the vacuum transmission characteristic providing a minimum I feedback in regenerative phase substantially at' the oscillation frequency and strong degenerative feedback at frequencies removed therefrom and including in itself means for controlling the amount of said feedback in response to variations of the amplitude oscillation so that staidtamplitude is maintained substantially cons an In one embodiment of the said prototype invention, this corresponding also to the single embodiment to be disclosed in the present specification, the feedback circuit comprises a Wheatstone bridge having its diagonals conjugately connected to the input and output circuits of the vacuum tube amplifier. One branch includes a frequency selective impedance, such as a piezoelectric crystal or a like mechanical vibratory element or the electrical analogue thereof, the other branches being constituted by resistances. At the oscillation frequency all four branches are resistive because-control is effected at the frequency where said impedance operates at or near series resonance. Since with perfect balance there would be no feedback, the circuit is operated with a very small amount of resistance unbalance. Because of the operation of tube with its grid negatively biased to such "an i extent that no grid current is drawn thereby eliminating the grid space path as a sourcev of wave form distortion. Because of the limitation to a relatively small amplitude of currentrtraversing the feedback path, the plate resistancev c: the tube is made substantially linear and theree fore non-productive of distortion. purity of the wave form is enhanced by the characteristic of the feedback circuit, which is such as to provide strong degenerative feedback at frequencies other than the oscillation frequency.

The feedback circuit, and therefore the elements principally distinguishing the invention of the prototype circuit as being responsible for the very great improvement along the lines indicated, is characterized by having a frequency selective Also the negligibly small.

the bridge principle, when the bridge is in a nearly balanced condition, any, phase shifts in the oscillator outside of the feedback network itself, and which would tend'to cause the over-all feed-back phase shift of zero degrees, .necessary for oscillation, to occur at some frequency slightly different from that of the-z resonance of the frequency selective bridge arm, would :be compensated in a greatly -magnified degree by the resultant change in condition ofsthe network. This is illustrated well by the vector diagram constituting Fig. 2 of the above patent disclosing the prototype circuit. Thereby the effect of phase-shift upon frequency stability is made Frequency stability is further enhanced by the use of an element in the frequency selective branch having a reactance that varies strongly with frequency about the resonance frequency of the branch. For that reason the degenerative feedback benefits by way of constancy of frequency and of wave form as well as of purity of waveform.

The circuit of the present invention differs (1) The network is so relatively electrically di- I mensioned that the mechanical vibrating element, piezoelectric crystal in the instance to be disclosed, vibrates at a much lower amplitude than has been used in previous circuits of this type, which means that a much smaller potential is impressed thereacross, the same being true if or when the electrical equivalent circuit is used in place of the mechanical vibrating element. Low amplitude operation is especially deampliiler. These phase shifts are in general of such a nature as to partially counteract thephase shifts produced by the various other reactances of the vacuum-tube amplifier, and in particular may be chosen to avoid approaching the conditions for oscillation at the frequency or frequencies where undesired singing would otherwise tend to occur. Thus at the expense sirable with certain types of resonators such as r the piezoelectric crystal for a number of reasons, including reduction in wear on the resonators and their mountings, reduction of heat dissipation therein, improved rigidityand permanency of mounting, and other improvements in the observed characteristics of these deviceswhich affect their short-time and long-time vstabilities.

To secure the above reduction of amplitude and at the same time to continue to. supply an ade.

quate amount of power to the thermally adjusted balance-controlling means (any known type of which is found to be insensitive to amplitude changes if the power dissipated therein is ex-.

tremely small), the network (bridge) has been given radically unequal arms or branches. The

inequality is such, in a practical instance, that the current through the lamp exceeds that through the crystal by a factor of about 16.

(2) As a bridge of unequal arms is less efllcient than one having four equal resistances, it is found that more amplification'is necessary in the oscillator, modified as in (1) above, if the same high sensitivity of balance, and accordingly the same degree of frequency and amplitude stabilization, is to'be secured as in the prototype-oscillater. In the above instant case, an increase of about. 20 decibels, is required infthe gain ofthe vacuum-tube amplifier, 'and'in fact still more gain is desirable to further improve the balance sensitivity and the stability; hence, a second stage of amplification is provided. r

(3) With large amounts of amplification, em-

ployed either for thereasons specified above or for any other reason, there is an inducement toward singing at unwanted frequencies, as will be brought out in the detailed description of the invention. Hence, an additional negative feedback is provided, entirely independently of any,

degenerative action of the bridge itself, to annul this tendency to sing. This is accomplished by a cathode network common to the input and out- 8 put circuits of one of theamplifier tubes (orby.

a plurality of such circuits so disposed in-rela tionto their respective tube's),, 'containing-,besides a resistor which shown. between is not critical except that it should inof a small decrease in amplification at the wanted oscillation frequency, desirable protective phase shifts are achieved at other frequencies.

In the accompanying drawings:

Fig. 1 illustrates an embodiment of the invention; and

Figs. 2 and 3 are so-called p diagrams in polar coordinates showing respectively certain attributes of the circuit of Fig. 1 without and with the additional features characterizing this circuit as compared with the prototype circuit.

The oscillation generator as a whole, as disclosed in Fig. 1, comprises an oscillator proper and a buffer amplifier, the latter, as its name implies, being adapted to function to prevent reaction from the load or output, indicated in the figure as connected to the output electrodes of said buffer amplifier, to the oscillator proper.

-This buffer amplifier is not an essential element of the amplifier and, therefore, including in order of energy transfer, the output transformer 3, a frequency selective network I having the characteristic pointed out in the statement of invention and here illustrated as a Wheatstone bridge, and an input transformer 5.

The choice of amplifier tube is not critical although perhaps the tube should preferably be of the screen-grid high amplification type as Likewise, the type of coupling theretroduce but little phase shift over the pass-band of the transformers 3 and 5. The drawing illustrates a conventional type of condenser-resistance coupling. In the particular oscillator contemplated throughout the specification as having been found very effective by experiment, the second stage of amplification was provided principally to make up for a 20 decibel loss, as compared with the prototype circuit, because of the bridge dissymmetry, this being an important feature of the invention. The power gain from the input to the low winding of the transformer corresponding to transformer -5 of Fig. l to the outputfrom the low winding of the transformer corresponding to transformer 3, terminated in determines the-maximum impedance of the network, an anti-resonant circuit, tuned to the'wanted frequency which is 100' kilocycles in the practical instance recited. At" "the network is, resistive,

this wanted frequency, w and introduces a small amountxofdegenerative feedbackwhich reduces the gain of theamplifier slightly without quencies,'ho'wever, the impedance ofthe cathode takes on a frequency variable-phase angle andintroduces correspond-i affecting'its phase. At otherdre V v .70 network is either capacitiveifor higher frequencieslior inductive (for lower frequencies); hence matching impedances, was decibe ls.

Energizing circuits for the anodes of the tubes are provided as shown. Theheatr circuits for the tube cathodes are shown in incomplete form. These heaters maybe energized, interconnected,

and connected to their associated circuitaall as in accordance with conventional practice between which there is a choice of alternatives. Negative biasing potentials for the control grids are obtained-from resistors 6 in the cathode leads,

7 which are traversed by the "plate current and from which alternating components thereof are effectually excluded in the case of the second' tube by the shunting capacitance shown. Prefcussed in another place.

- may operate substantially at exact reactive balance. To that end the transformers may be tuned by use of capacitors 1 which may be adjusted so that the transformer phase shifts are substantially zero at the desired frequency.

Consistent with its sole function of preventing reaction from the load circuit connected to its output to the anterior oscillator proper, the buffer amplifier shown at the right may be of a choice of conventional types. For convenience and as shown, it may be of the same general type as the unit amplifiers of the oscillator proper. As is evident from the drawings, it obtains its plate energization and grid bias in a similar manner, and it may be coupled to the load, in a similar manner, through a tuned circuit as shown.

The feedback network 4 is, in the specific form disclosed, a true Wheatstone bridge. Specifically the bridge, at the operating point of the crystal 8, which controls the frequency, is comprised wholly of pure resistances except to the extent that the reactance branch or armconsisting of the crystal 8 and series impedance elements 9 and I might deviate slightly from the optimum series resonance condition, as might result, for example, by phase changes in the amplifier or in the transformers, in fact anywhere exteriorly of the bridge network itself. Under ideal conditions with zero phase shift in the transformers and in the coupling between the tubes so that the round trip path exteriorly of the bridge is effectively reactanceless, the reactance arm would operate at series resonance so as to effectively constitute a pure resistance. The principles of operation, as to this feature, are the same as in the circuit of applicant's patent above identified and the description therein, especially with reference to the vector diagram of Fig. 2 therein, is believed to be adequate.

Two opposite arms of the bridge network comprise resistors R2 and Rs. A third arm comprises a variable resistance device R1 such as the filament of a lamp, that is a resistance which has a pronounced positive temperature coefficient. The fourth arm, as above explained, comprises a crystal 8 and impedance elements 9 and I0. One pair of diagonally opposite corners A and C are connected to the terminals of input transformer 5, the other corners B and D being connected correspondingly to the terminals of output transformer 3. The requisite phase of the feedback production of oscillation, other things being equal, may be obtained by poling the connections of one or other of the transformers or by interchanging the connections of the bridge corners. Since the operating point of the crystal will tend automatically to vary from series resonance conditions to compensate for extraneous phase shifts, as indicated by Fig. 2 of applicant's patent, above mentioned, the use of series impedances 9 and If), each of which is variable as indicated, is not necessary in all instances. However, these impedance elements are capable of the conventional function of a so-called trimming crystal.

impedance to effectively slightly change the control frequency otherwise determined solely by the In the performance of this function a single type of reactance instead of the combinationof the two types in series, could be used although with somewhat less flexibility, or said trimming impedance portion of the bridge network could be connected in shunt instead of in series with the crystal whether one or both types of reactance are used- The operation of the bridge principle, as is evident from the disclosure in the applicant's previous patent, supra, is most effective when the bridge is most nearly balanced because with this condition the maximum variation of magnitude and phase of the fed back potential occurs with a given change of impedance of a bridge arm. Such change in bridge arm impedance might result for example by the control action of the crystal when changing from its optimum condition of series resonance, corresponding to zero reactance, to a finite reactance, responsive to a change in outside conditions. The amount of the feedback is, of course, dependent on the degree of the bridge unbalance, but byusing a high gain amplifier sufficient feedback to maintain oscillations can be obtained with a very small unbalance.

These various considerations, in the instance of the previous bridge oscillator, had dictated the use of a fairly symmetrical bridge, that is, a bridge with nearly equal arms, the said previous oscillator differing radically from the present considered oscillator in that particular. The necessity of using for arm R1 a resistance variable with temperature, within practical limitations of choice, imposes certain requirements on the bridge, affecting the matter of symmetry. In the case of the previous oscillator, the necessary conditions were easily satisfied. That is, with the use of an effective type of element for R1, the bridge could be balanced with a desirable degree of symmetry and with a resultant efficient potential on the crystal for effective drive, it being found further that an ample degree of amplification using a single tube was possible, having in mind the maximum stability then perceived to be required. The situation differs, in the instant case, because of the appreciation of the very great benefit accruing from driving the crystal at an amplitude very much less than was used before.

In order to bring about this low amplitude of crystal vibration, hence low amplitude of potential impressed across the crystal bridge arm, with approximately the same value of thermally responsive resistance of resistor R1 and of course assuming a condition of near balance, it is neces sary to make the resistance of R2 very small as compared with the resistance of R3. In the practical instance cited several times in this specification, a typical value of R2 was 2.8 ohms and that of R3 was 2,000 ohms. It will be recalled that in the previous bridge stabilized oscillator these resistances had substantially the same value. The ultimate result is that with such radically unlike arms more gain is necessary to secure the same sensitivity of balance. This condition provides the necessity for the second stage of amplification. Actually in the circuit tested, the bridge dissymmetry resulted in an effective reduction of 20 decibels in the gain of the amplifier circuit. The second stage of amplification not only made up this loss but provided a certain additional gain, the power gain from the input to the low winding of the input transformer to the output of the low winding of the output transformer being '70 decibels.

Analysis of the purpose and function of the cathode network, consisting of elements 6, II, I2, I3 and I4, by which a special negative feedback about tube I is induced, will be facilitated by preceding it with a discussion of Figs. 2 and in the round trip path. This type of diagram I is old in the art under the common denomination p diagram, Examples are given in the paper by H. S. Black Stabilized Feedback Amplifiers in the January, 1934, issue of the Bell System Technical Journal. In a general way the information derivable from these Figs. 2 and 3 diagrams is comparable as to kind and extent with that derivable from the vector diagram of Fig. 2 of the previous patent although the latter described merely the operation of the bridge, whereas the p diagrams deal with the entire circuit.

Fig. 2 shows the m8 characteristic of a bridge stabilized oscillator having a relatively small amount of gain and therefore requiring no phasecorrecting cathode network. It therefore implies a circuit like that of the previous patent. It discloses the general form of the locus and is useful as teaching that for increasing frequency the values of s follow the particular sequence illustrated. That is, for increasing frequency, the end of the m3 vector follows the path abcdefgha, covering the portion abc as the frequency changes from zero to very nearly that of the crystal (a function of the properties of the output and input transformers), the portion cdefg in an exceedingly narrow frequency band (determined largely by the bridge and corresponding as to frequency range with the circle locus of Fig. 2 of the previous patent) and the portion gha in the range from slightly above the crystal resonance to infinity (again a function of the transformers). According to theory well known in the art, oscillation commences if the loop encloses the point having the coordinate I, 0 as indicated onthe figure. When they amplitude is small and the lamp R1 is cool, the bridge is so unbalanced that the loop does enclose this point as indicated by the broken locus st. As the amplitude increases the lamp filament becomes warmer, moving the locus st to the left until it passes exactly through said point I, 0. The frequency represented by the point on this curve which then coincides with I, I) is the frequency of oscillation. In the Fig. 2 diagram, the portion def of the locus is the only portion which can enclose this point; hence there is no danger of an undesired oscillation at someother frequency than the one above mentioned, this portion def being a part of the inner loop responsive to control by the crystal and the thermally-adjusted resistor R1. y

The Fig. 3 diagram is similar to that of Fig. 2, the difference being that it assumes conditions corresponding to that of the present invention, that is, with the value of a greatly increased by the use of an additional stage or stages of amplification. It results that with the diagram drawn to the same scale as Fig. 2, much ofthe locus is outside the boundary of the drawing. As before, andnow referring to the solid curve, the lamp should adjust the portion of the curve def to pass through the point I, I) exactly without complication as by the incidence of conditions tending to cause the curve as a whole to have another crossing point, for instance, at I, 0 or on the zero axis outside that point. However, with the circuit dlflering from that con templated by Fig. 2 only by the use of an increased this is impossible because with the curve magnified by increased gain, the point I: at which it crosses the zero axis is on the righthand side of point I, 0, so that the curve as a whole encloses the I, II point twice. In such an instance, the circuit is found to oscillate at a high frequency represented by point k which, however, is actually moved leftward to coincide with said point I, 0 by the ultimate decrease in a resulting .from tube overloading, since there can be no stable oscillations except at this point I, 0. Oscillation at a point on the portion def may also occur, but with reduced stability because of loss of gain, the presence of harmonics and the heating effect of the unwanted high frequency current, which may vary from time to time in frequency and amplitude, in the balancecontrolling lamp.

At the low-frequency end ab of the an locus, the point I, 0 is not quite enclosed but there is very little phase margin; hence, some minor change in the circuit might lead to enclosure, accompanied by further undesired oscillations. It should be realized that the form of the curve in Fig. 3 may be varied considerably by the design of the transformers and other circuit elements, and that either or both of the portions ab and ha may intersect the axis of zero phase to the right of point I, 0, at points such as k. Fig. 3 merely illustrates one form of the curve, which happens to apply to the particular experimental embodiment referred to previously.

The cathode network is designed to introduce phase shifts at both the high and low frequency ends of the locus, which move the critical parts back away from the point I, II as shown by the broken curve .in this Fig. 3, thus protecting against extraneous frequency singing. While this is done at the expense of a small decrease in gain (about 3 decibels in the particular circuit tested) which effectively reduces the diameter of the inner loop def, the additional amplification which the protection against singing makes possible is /'substantially greater than this loss; thus an improvement in net gain and in frequency stability may be utilized.

The functions of the various elements of this cathode network are as follows: The resistant 6, traversed in particular by the direct current component of the plate current of tube I, provides bias between the grid and cathode of this tube. The inductance II and the "capacitance I2 are anti-resonant at the wanted frequency of the oscillator, and have the function of providing reactances of suitable magnitudes inthe cathode path at other frequencies, in order to'afford, by

means of local feed-back about tube I, desirable phase shifts in the transmission about the OSCfllfl',

from being short-circuited by the inductance- I I.

The resistance I4, in parallelwith resi'stance'IL determines the magnitude of the negative feedback at the operating or wanted frequency;

not introduce any phase shift. Furthermore, at

very much higher frequencies, the reactance of capacitance I2' is small and the impedance in the cathode circuit is very low; hence there is shift is introduced. At some intermediate frequency, therefore, the reactive component of the impedance of the network must reach a maximum value, and at about this same frequency must produce its greatest phase-shifting effect by the local feedback. Similar remarks also apply to the inductance H at certain frequencies lower than that of anti-resonance. Although elements II and I2 may not be chosen independently if they are to anti-resonate at the wanted oscillation frequency, nevertheless they may be chosen together, so that the two frequencies of maximum phase shift, one higher and one lower than the operating value, are most effectively situated with respect to the critical frequency ranges which are represented in Fig. 3. by the passage of portions ab and ah near the point I, 0. That is, the elements of the cathode network may be designed to give their maximum protection against undesired singing at or near the frequencies for which protection is needed. It should be obvious that this protection, as being with reference to frequencies outside the range of control by the feedback network 4, and therefore outside the locus 20 of Fig. 2 of applicant's previous patent, hereinbefore mentioned, cannot be secured by the degenerative action of. said network 4 and therefore is impossible of accomplishment either in degree or in kind by the prototype circuit of said patent.

In concluding this specification it may be well to point out again that the large improvements over the prototype bridge oscillator which have been realized by means of the present invention have resulted partly from the-physical benefits obtained by operating a mechanical resonator at extremely low amplitudes, and partly from increased amplification, which according to the bridge oscillator theory results in more sensitive stabilization, and which would be impracticable to utilize (owing to unavoidable phase shifts in the prototype oscillator) without the phasecorrecting means disclosed herein.

While the invention has been described in connection with a particular embodiment, certain variations have been suggested and it is to be understood that many additional changes are possible within the scope of the invention, these changes in particular being directed to other forms of balanced feedback circuits.

What is claimed is:

1. In an oscillation generator comprising an amplifier and a feedback path coupling the outlittle local feedback, and practically no phase put and input terminals thereof, a conjugate network included in said feedback path and connected at its conjugately related pairs of points, through the remaining portion of said feedback path if any, to said output and input terminals, respectively, said network including a frequency determining element and having a transmission characteristic providing a minimum feedback in regenerative phase substantially at the oscillation frequency and strong degenerative feedback frequencies removed therefrom, said frequency determining element comprising a mechanical vibratory means and. the electrical elements of said network being so relatively dimensioned by asput and input terminals thereof,

signment of radically different impedance values to certain functionally discrete portions thereof that said vibratory element tends to vibrate at a relatively low. amplitude as measured by the condition of the rate of change of frequency with amplitude becoming a mimmum.

2. An oscillation generator comprising an amplifier, input and output circuits therefor, and a Wheatstone bridge network conjugately coupling said input and output circuits, said network comprising a mechanical vibratory frequency determining element, the electrical dimensions of said network being such by radical disparity of impedance values in the network arms that said element tendsto vibrate at an amplitude corresponding to a condition of the rate of change of frequency with amplitude becoming a minimum.

3. In an oscillation generator comprising an amplifier and a feedback path coupling the outa conjugate network included in said feedback path connected at its conjugately related pairs of points, through the remaining'part of said feedback path if any, to said output and input terminals, respectively and comprising a piezoelectric crystal element, said network having a transmission characteristic providing a minimum feedback in regenerative phase substantially at the oscillation frequency and strong degenerative feedback at frequencies removed therefrom, the electrical elements of said network also being so relatively electrically dimensioned by assignment of radically different impedance values to functionally discrete portions thereof as to provide a potential across said crystal element corresponding substantially to a condition of the rate of change of frequency with amplitude of vibration of said crystal as determined by said potential becoming a minimum.

4. An oscillation generator comprising an amplifier, input and output circuits therefor, and a Wheatstone bridge network conjugately coupling said input and output circuits, said bridge network comprising a piezoelectric crystal element, the relative electrical dimensions of said network by radical departure from uniformity of impedance values in the respective bridge arms and the electrical dimensions of said amplifier being such that said crystal element tends to vibrate at as small an amplitude as is consistent with the condition of a minimum rate of change of frequency with amplitude.

.5. In an oscillation generator comprising an amplifier and a feedback path coupling the output and input terminals thereof, a conjugate network included in said feedback path connected at its conjugately related pairs of points, through the remaining part of said feedback path if any, to said output and input terminals, respectively and comprising a piezoelectric crystal element and a resistor the resistance of which is variable in magnitude with the strength of the current traversing it, said network providing a minimum feedback in regenerative phase substantially at the oscillation frequency and strong degenerative feedback at frequencies removed therefrom, the relative electrical dimensions of said network by assignment of radically different impedance values to functionally discrete impedance elements thereof being such that the energy dissipation in said crystal element is small as compared with that in said resistor, and the electrical dimensions of said amplifier in relation to those of said network being such as to maintain a desired degree of network balance.

6. The oscillation generator specified in claim 5 including additionally a negative feedbackv means comprising a circuit anti-resonant to the desired frequency in common to an input and an output path of the amplifier, whereby to introduce protective phase shifts for the avoidance of singing at extraneous frequencies induced by a high degree of amplification.

8. An oscillation generator comprising an amplifier, input and output circuits therefor, and a Wheatstone bridge network coupling said input and outputcircuits, the bridge comprising in one arm a piezoelectric crystal element, and in another arm a resistor whose resistance is thermally responsive to the current flowing therethrough, the remaining two arms having greatly different values of resistance whereby, for a near-balanced condition of the bridge,the energy dissipation in crystal element is small as compared with that in said resistor, an amplifier having as large a gain as may be necessary to compensate for the inefficient feedback to the input thereof, consequent on the resultant dissymmetry of the bridge, and a negative feedback means comprising a circuit anti-resonant at the desired frequency in a lead common to an input and an output circuit of the amplifier, for introducing protective phase shifts with respect to singin frequencies induced by the use of a larger amphfier gain than would be necessary with a more nearly symmetrical bridge which would result in turn from a normally increased amplitude of vibration of the crystal.

9. An oscillation generator comprising, an amplifier, a feedback path coupling the output and input terminals thereof, a frequency determining means being included in said feedback'path, and,

independently of said feedback path, a negative feedback means comprising .acircuit anti-resonant to the desired oscillationfrequency in common to an input and an output path of the amplifier, said anti-resonant circuit being so electrically dimensioned as to have a strong reactive feedback at one or more desired frequencies whereby to introduce protective phase shift for the avoidance of singing atsaid frequencies while the anti-resonant character of the circuit inlsures the lack of phase shift at aid desired frequency. I

10. An oscillation generator comprising, an amplifier, a feedback path coupling the output and input terminals thereof, a network being included in'said feedback path and comprising a frequency determining means and a resistor the resistance of which is variable in magnitude with the strength of the current traversing it, said network providing a minimum feedback in 'regenerative phase substantially" at the oscillation frequency and strong degenerative feedback at frequencies removed therefrom, and a negative feedback means associated with said amplifier and so electrically dimensioned as to insure that oscillation at any frequency other than that which is controlled and determined by the frequency determining element is effectively prevented.

11. The oscillation generator specified in claim 1 10 in which the frequency determining means is a piezoelectric crystal element.

12. The oscillation generator specified in claim 10 in which said negative feedback means com prises a circuit anti-resonant to the desired frequency in common to an input and an output path of the amplifier and so electrically dimensioned as to have a strong reactive feedback,

with attendant phase shift, at one or more frequencies at which the oscillator tends to sing.

LARNED A. MEACHAM. 

